Resonant converters with variable inductor

ABSTRACT

Unique systems, methods, techniques and apparatuses for a ZVT ZCT resonant converter with a variable resonant tank are disclosed. One exemplary embodiment is a system comprising a bidirectional resonant converter comprising an input/output terminal, a switching device coupled with the input/output terminal, a resonant circuit coupled with the switching device and including a variable inductor, an output/input terminal coupled with the resonant circuit, and a DC biasing circuit operatively coupled with the variable inductor. The variable inductor comprises a toroidal core, a first winding wound around the toroidal core and coupled with the switching device and the output/input terminal, a second core structured to overlap a portion of the toroidal core, and a second winding wound around the second core and coupled with the DC biasing circuit. The DC biasing circuit is controllable to vary the inductance of the variable inductor by saturating a portion of the toroidal core.

BACKGROUND

The present disclosure relates generally to resonant converters.Resonant converters, including zero-voltage transition (ZVT) pulse widthmodulation (PWM) converters and zero-current transition converters (ZCT)PWM converters offer a number of benefits including, for example,allowing high switching frequencies, reducing electromagnetic noiseemission, and allowing use of smaller passive components. Existingresonant converters suffer from a number of shortcomings anddisadvantages. There remain unmet needs including increasing resonanceoperation range, reducing switching losses, and decreasing stress onconverter components. For example, when resonant converters operateoutside their resonant operating range, the converter switching lossesincrease and the stress on the switches increase. There is a significantneed for the unique apparatuses, methods, systems and techniquesdisclosed herein.

SUMMARY

For the purposes of clearly, concisely and exactly describingnon-limiting exemplary embodiments of the disclosure, the manner andprocess of making and using the same, and to enable the practice, makingand use of the same, reference will now be made to certain exemplaryembodiments, including those illustrated in the figures, and specificlanguage will be used to describe the same. It shall nevertheless beunderstood that no limitation of the scope of the present disclosure isthereby created, and that the present disclosure includes and protectssuch alterations, modifications, and further applications of theexemplary embodiments as would occur to one skilled in the art with thebenefit of the present disclosure.

Exemplary embodiments include unique systems, methods, techniques andapparatuses for a zero-voltage transition zero-current transitionresonant converter with a variable resonant tank. Further embodiments,forms, objects, features, advantages, aspects and benefits of thedisclosure shall become apparent from the following description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary resonant converter.

FIGS. 2-5 illustrate exemplary variable inductors.

FIG. 6 illustrates another exemplary resonant converter.

FIG. 7 is a plurality of graphs illustrating the resonant operatingrange of an exemplary resonant converter, such as the converters ofFIGS. 1 and 6.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

With reference to FIG. 1 there is illustrated an exemplary resonantconverter 100. It shall be appreciated that converter 100 may beimplemented in a variety of applications, including electric vehicles,hybrid vehicles, motor drives, and medium voltage power distribution toname but a few examples. In the illustrated embodiment, converter 100 isstructured to convert DC power to three phase AC power. In otherembodiments, converter 100 is structured to convert DC power at onevoltage level to DC power at a second voltage level or DC power tosingle phase AC power.

Converter 100 is coupled to a DC power supply 101 having a positive poleand a negative pole. Specifically, the power supply 101 is coupled to aDC bus 103 having a positive rail 105 and a negative rail 107. Thepositive rail 105 of DC bus 103 is coupled to the positive pole of theDC power supply 101 and the negative rail 107 of the DC bus 103 iscoupled to the negative pole of the DC power supply 101. The voltagedifference between the positive rail 105 and the negative rail 107 isV_(dc). It shall be appreciated that power supply 101 may be coupleddirectly to DC bus 103 or may be coupled to DC bus 103 via one or moreintermediate components and that the same is true of other elementsdescribed here as being coupled to or with another element unlessexpressly indicated to the contrary.

Converter 100 further includes an auxiliary circuit 110 having threelegs corresponding to three converter phases. It shall be appreciatedthat other embodiments my utilize converters with a different number oflegs corresponding to a different number of converter phases rangingfrom as few as one phase/leg to greater than three phases/legs. Thefirst leg includes a first switching device 111 a having a firstterminal coupled to positive rail 105 and a second terminal coupled to aresonant node 115 a. In the illustrated embodiment, switching device 111a is an insulated gate bipolar transistor (IGBT) having a freewheelingdiode. Switching device 111 a may be any type of semiconductor switchingdevice. It shall be appreciated that any or all of the foregoingfeatures of switching device 111 a may also be present in the otherswitching devices disclosed herein.

The first leg of circuit 110 also includes a second switching device 113a having a first terminal coupled to resonant node 115 a and a secondterminal coupled to negative rail 107. The second leg includes a firstswitching device 111 b having a first terminal coupled to the positiverail 105 and a second terminal coupled to a resonant node 115 b. Thesecond leg also includes a second switching device 113 b having a firstterminal coupled to the resonant node 115 b and a second terminalcoupled to the negative rail 107. The third leg includes a firstswitching device 111 c having a first terminal coupled to positive rail105 and a second terminal coupled to a resonant node 115 c. The thirdleg also includes a second switching device 113 c having a firstterminal coupled to a resonant node 115 c and a second terminal coupledto negative rail 107.

Converter 100 further includes a variable resonant tank 120 coupled toresonant nodes 115 a-115 c. Specifically, tank 120 includes aseries-coupled first variable inductor 121 a and a first capacitor 123 acoupled to resonant node 115 a; a series-coupled second variableinductor 121 b and a second capacitor 123 b coupled to resonant node 115b; and a series-coupled third variable inductor 121 c and a thirdcapacitor 123 c coupled to resonant node 115 c. In certain embodiments,capacitors 123 a-123 c are variable capacitors. As discussed in moredetail below, variable inductors 121 a-121 c include a toroidal core; afirst winding wound around the toroidal core and coupled with theauxiliary circuit 110 and a primary circuit 130; a second corestructured to overlap a portion of the toroidal core; and a secondwinding wound, also known as a DC biasing coil, around the second coreand coupled with a controller 150, also known as a DC biasing circuit.

The primary circuit 110 includes three legs. The first leg includes afirst switching device 131 a having a first terminal coupled to positiverail 105 and a second terminal coupled to an output node 135 a. Thefirst leg also includes a second switching device 133 a having a firstterminal coupled to output node 135 a and negative rail 107. The secondleg includes a first switching device 131 b having a first terminalcoupled to the positive rail 105 and a second terminal coupled to anoutput node 135 b. The second leg also includes a second switchingdevice 133 b having a first terminal coupled to the output node 135 band a second terminal coupled to the negative rail 107. The third legincludes a first switching device 131 c having a first terminal coupledto the positive rail 105 and a second terminal coupled to an output node135 c. The third leg also includes a second switching device 133 chaving a first terminal coupled to the output node 135 c and a secondterminal coupled to the negative rail 107.

Output node 135 a is coupled to capacitor 123 a of resonant tank 120 andan output line 140 a; output node 135 b is coupled to capacitor 123 band an output line 140 b; and output node 135 c is coupled to capacitor123 c and an output line 140 c. Output nodes 135 a-135 c are coupled toa load structured to receive three phase AC power.

Controller 150 is coupled to switching devices 111 a-111 c, 113 a-113 c,131 a-131 c, and 133 a-133 c as well as variable inductors 121 a-121 c.Controller 150 is structured to operate the switching devices bytransmitting activation signals to each switching device. Controller 150is also structured to vary the inductance value of the variable inductorby saturating a portion of the toroidal core with magnetic flux. Bysaturating a portion of the toroidal core, the inductance of thevariable inductor is reduced. Controller 150 saturates the toroidal coreby providing DC power to the DC bias coil of the variable inductor.Controller 150 may be one controller or a plurality of controllers. Itshall be appreciated that any or all of the foregoing features ofconverter 100 may also be present in the other converters disclosedherein.

With reference to FIG. 2 there is illustrated an exemplary variableinductor 200. In one embodiment, inductor 200 is one of the variableinductors 121 a-121 c of converter 100 in FIG. 1. Inductor 200 includesa toroid 201 having an external surface. In the illustrated embodiment,the toroid is structured as a torus with a circular cross section. Inother embodiments, toroid 201 is a hollow square section ring with arectangular cross section. A first winding 203 is wound around a portionof the external surface of the toroid 201. One end of winding 203 iscoupled to an AC power source, and the other end of the winding 203 iscoupled to an output node. As AC power flow through winding 203, AC fluxis generated in a toroidal flux path. Toroid 201 may include an air gappositioned such that the air gap and the first winding 203 overlap alonga portion of the toroid 201. A core 205 is located in the center of thetoroid 203 such that the toroid 201 encircles the core 205 and a secondwinding 207, also known as a DC bias coil, is wound around core 205. Incertain embodiments, the core 205 may include an air gap. A DC powersource is coupled to core 205. When DC power flows through winding 207,a DC flux is generated along a DC flux path perpendicular to the AC fluxpath. The DC flux saturates a portion of the AC flux path. Since the DCflux path has no air gap and the AC flux path has an air gap, saturatingthe toroid 201 using the second winding 207 requires less power, whereasenergy can be stored in the air gap in the AC flux path, leading toreduced inductor size.

In the illustrated embodiment, core 205 includes a cylindrical portion.In other embodiments, the core 205 includes a rectangular portion or asquare portion, to name a few examples. The core may also include twoplates 207 a and 207 b placed on a portion of the external surface ofthe toroid 201 and an additional two plates 209 a and 209 b are placedon plates 207 a and 207 b, respectively, such that plates 209 a and 209b are in contact with the cylindrical core portion 205. A first fluxpath through the toroid 201 for the first winding 203 is generallytoroidal and a second flux path through the toroid 201 for the secondwinding 207 is perpendicular to the first flux path.

In certain embodiments, inductor 200 includes a second core structuredto overlap a second portion of the toroid 201, and a second magneticwinding wound around the second core and coupled with a second DCbiasing circuit, the second DC biasing circuit being controllable tovary the inductance of the variable inductor by saturating a secondportion of the toroid 201 with magnetic flux. The second core may havean external surface, a first end, a second end, a second magneticwinding wound around the external surface of the second cylinder, asecond top plate coupled to the first end of the second cylinder and incontact with the external surface of the toroid, and a second bottomplate electrically coupled to the second end of the second cylinder andin contact with the external surface of the toroid 201.

Inductor 200 may be incorporated into a resonant converter, such asconverter 100 of FIG. 1. With continuing reference to FIG. 1, controller150 is structured to generate a current through the variable inductor200 by operating the switches of auxiliary circuit 110; provide acurrent to the second winding 207 of the variable inductor so as toalter the inductance of the variable inductor 200 with the controller150; provide power to the second terminal of one of the switchingdevices 131 a-131 c with a current and voltage substantially equal tothe current and voltage of the power provided to the first terminal ofthe same switching device; and open the second switching device duringthe zero-voltage, zero-current condition. The controller 150 isadditionally structured to receive voltage and load currentrequirements; calculate a desired resonant tank impedance using thevoltage and load current requirements; and provide a current to thesecond winding of the variable inductor with the controller in responseto the calculating the desired resonant tank impedance.

The controller 150 is structured to operate the switching devices of theauxiliary circuit 110 by opening and closing the switching device atresonant frequency so as to provide a resonant current to the outputnodes 235 a-235 c. For each output phase, the resonant frequency isdetermined by the inductance value of the variable inductors 121 a-121 cand the capacitance of the capacitors 123 a-123 c using the followingequation, where f₀ is resonant frequency, L_(x) is inductance of thevariable inductor and C_(x) is the capacitance of the capacitor:

$\begin{matrix}{f_{0} = \frac{1}{\left. \sqrt{}L_{x} \right.C_{x}}} & (1)\end{matrix}$

For embodiments where the variable inductor 200 includes more than onecore 205, the controller 150 is additionally structured to provide acurrent to the second magnetic winding of the variable inductor 200 inresponse to calculating the desired resonant tank impedance. Forexample, the controller 150 may use a lookup table to determine whetherto provide a current through the magnetic winding. It shall beappreciated that any or all of the foregoing features of variableinductor 200 may also be present in the other variable inductorsdisclosed herein.

With reference to FIG. 3 there is illustrated an exemplary variableinductor 300. FIG. 3a is a top view of inductor 300 and FIG. 3b is aside view of inductor 300. Inductor 300 includes a toroid 301 having anexternal surface. A first winding 303 is wound around a portion of theexternal surface 303 of the toroid 301. A core 305 is located on theoutside of the toroid 301 such that a portion of the core 305 is incontact with the external surface of the toroid 301. In the illustratedembodiment, core 305 is structured as a C-shaped structure including aportion overlapping the toroid 301, a middle portion around which asecond winding 301 is wound, and a third portion overlapping the toroid301, also known as a C-core.

With reference to FIG. 4(a) there is illustrated an exemplary variableinductor 400 including a toroid 401 having an external surface. Theinductor 400 also includes a first winding 403 wound around a portion ofthe external surface of the toroid 401. A C-core is located on theinside of the toroid such that a portion of the C-core 405 is in contactwith the toroid 401. A second winding 407 is wound around the C-core405.

With reference to FIG. 4(b) there is illustrated another exemplaryvariable inductor 410 including a toroid 411 having an external surface.A first winding 413 is wound around a portion of the external surface ofthe toroid 411. A first C-core 405 is located on the outside of thetoroid 411 such that C-core 415 is in contact with the toroid 411. Asecond winding 417 is wound around C-core 415. A second C-core 419 islocated on the outside of the toroid 411 such that C-core 415 is incontact with the toroid 411. A third winding 421 is wound around C-core415. A third C-core 423 is located on the outside of the toroid 411 suchthat C-core 423 is contact with the toroid 411. A fourth winding 417 iswound around C-core 415.

With reference to FIG. 5(a)-(c) there are side views illustratinganother exemplary variable inductor 500 having a toroid 501; a firstwinding 503 wound around toroid 501; a C-core 505 located on the outsideof the center of toroid 501; and a second winding 507 wound aroundC-core 505. C-core 505 may be located a varying distance 509 a, 509 b,or 509 c from toroid 501. Distance 509 a-509 c may be adjusted duringoperation of the inductor 500 so as to vary the saturation of the ACflux path of inductor 500.

With reference to FIG. 6 there is illustrated an exemplary converter 600structured to convert DC power at a first voltage to DC power at asecond voltage. Converter 600 is coupled to a DC power source 601 havinga positive pole and a negative pole. Converter includes a DC buspositive rail 605 coupled to the positive pole and a DC bus negativerail 607 coupled to the negative pole. The voltage difference betweenpositive rail 605 and negative rail 607 is V_(dc). Converter 600includes an auxiliary leg 610 having a first switching device 611 with afirst terminal coupled to positive rail 605 and a second terminalcoupled to a resonant node 615. Leg 610 also includes a second switchingdevice 613 having a first terminal coupled to the resonant node 615 anda second terminal coupled to the negative rail 607.

Converter 600 also includes a resonant tank 620 having a variableinductor 621 and a capacitor 623 coupled in series to resonant node 615.Converter 600 includes a primary leg 630 having a first switching device631 with a first terminal coupled to positive rail 605 and a secondterminal coupled to an output node 635. Leg 630 also includes a secondswitching device 633 having a first terminal coupled to the output node635 and a second terminal coupled to negative rail 607. Output node 635is coupled to resonant tank 620 and a load 640.

Converter 600 further includes controller 650 coupled to switchingdevices 611, 613, 631, and 633, as well as variable inductor 621.Controller 150 is structured to operate the switching devices bytransmitting activation signals. Controller 150 is structured to varythe inductance value of the variable inductor 621 by transmitting DCpower to the variable inductor 621.

With reference to FIG. 7 there is illustrated a plurality of graph s 700including graph 710 illustrating the operating range of a resonantconverter with a fixed resonant tank. Graph 710 illustrates a hardswitching operating range 711 where normal hard switching has a lowerloss than the soft switching using resonance, such as in light loadconditions; a resonant operating range 713 where soft switching usingresonance reduces switching losses; a quasi-resonant operating range 715where soft switching using resonance reduces switching losses, but thereis reduced current turn-on due to operation limitations; and a hardswitching operation range 717 where soft switching is not possible dueto operational limitations.

Graph 720 illustrates the resonant operating range of a resonantconverter with a variable resonant tank, such as the variable resonanttank of FIGS. 1 and 6. Graph 720 illustrates a hard switching operatingrange 721 where normal hard switching has a lower loss than the softswitching using resonance; a plurality of expanded resonant operatingranges 723 where soft switching using resonance reduces switchinglosses; and a plurality of quasi-resonant operating ranges 725 wheresoft switching using resonance reduces switching losses, but there isreduced current turn-on due to operation limitations.

Further written description of a number of exemplary embodiments shallnow be provided. One embodiment is a system comprising a bidirectionalresonant converter comprising an input/output terminal, a switchingdevice coupled with the input/output terminal, a resonant circuitcoupled with the switching device and including a variable inductor, anoutput/input terminal coupled with the resonant circuit, and a DCbiasing circuit operatively coupled with the variable inductor; whereinthe variable inductor comprises a toroidal core, a first winding woundaround the toroidal core and coupled with the switching device and theoutput/input terminal, a second core structured to overlap a portion ofthe toroidal core, and a second winding wound around the second core andcoupled with the DC biasing circuit; and wherein the DC biasing circuitis controllable to vary the inductance of the variable inductor bysaturating a portion of the toroidal core with magnetic flux.

In certain forms of the foregoing system, the second core comprises acylindrical portion positioned within the center of the toroidal coreand top and bottom plate portions extending radially outward to overlapa portion of the toroidal core. In certain forms, the second corecomprises a C-shaped structure including a top portion overlapping thetoroidal core, a middle portion around which the second winding iswould, and a third portion overlapping the toroidal core. In certainforms, the variable inductor comprises a third core structured tooverlap a second portion of the toroidal core, and a third winding woundaround the third core and coupled with a second DC biasing circuit, thesecond DC biasing circuit being controllable to vary the inductance ofthe variable inductor by saturating a second portion of the toroidalcore with magnetic flux. In certain forms, a first flux path through thetoroidal core for the first winding is generally toroidal and a secondflux path through the toroidal core for the second winding isperpendicular to the first flux path. In certain forms, the toroidalcore comprises an air gap positioned such that the air gap and the firstwinding overlap along a portion of the toroidal core.

Another exemplary embodiment is a variable inductor comprising a toroidcomprising a first external surface; a first winding wound around atleast a portion of the first external surface, the winding being coupledto an AC power source to generate AC flux in an AC flux path; a corecomprising a second external surface; and a second winding wound aroundthe second external surface of the core, coupled to a controllerproviding DC power to the second winding, and structured to generate DCflux in a DC flux path, the DC flux path passing through a portion ofthe toroid, wherein the presence of DC flux generated by the secondwinding alters an inductive value of the inductor.

In certain forms of the foregoing system, the core is structured to beselectively moveable relative to the toroid effective to selectivelyvary the portion of the toroid through which the DC flux passes. Incertain forms, the controller is structured to selectively saturate theAC flux path by providing DC power to the second winding. In certainforms, the core comprises a cylinder with a first end and a second end,a top plate coupled to the first end of the core and in contact with thefirst external surface of the toroid, and a bottom plate coupled to thesecond end of the core and in contact with the first external surface ofthe toroid. In certain forms, the inductor comprises a second corehaving a cylinder with a third external surface, a first end, and asecond end, a second DC winding wound around the third external surfaceof the cylinder of the second core, a second top plate coupled to thefirst end of the cylinder of the second core and in contact with thefirst external surface of the toroid, and a second bottom plateelectrically coupled to the second end of the cylinder of the secondcore and in contact with the first external surface of the toroid. Incertain forms, a portion of the core is encircled by the toroid. Incertain forms, the toroid is structured as one of a torus with acircular cross section or a hollow square section ring with arectangular cross section. In certain forms, the core comprises aC-core.

A further exemplary embodiment is A method comprising operating resonantconverter circuitry including a first switching device operativelycoupled with a first input/output terminal and a resonant tank, theresonant tank including a variable inductor, the variable inductorincluding a first core, a first winding wound around the first core andoperatively coupled with the first switching device, a second core, anda second winding wound around the second core, a second switching deviceoperatively coupled with the first winding of the resonant tank and asecond input/output terminal, and an electronic control systemoperatively coupled with the first switching device and the secondswitching device and structure to control current to the second winding;operating the electronic control system to provide a current through thefirst winding of the variable inductor, provide a current through thesecond winding of the variable inductor so as to alter the inductance ofthe variable inductor, provide power to the second switching device fromthe resonant tank such that the voltage and current applied to secondswitching device by the resonant tank are with a current and voltagesubstantially equal to the current and voltage provided to the secondswitching device by the second input/output terminal, and open thesecond switching device during the zero-voltage, zero-current condition.

In certain forms of the foregoing method, the method comprises receivingvoltage and load current requirements with the electronic controlsystem; calculating with the electronic control system a desiredresonant tank impedance using the voltage and load current requirements;and operating the electronic control system to provide a current to thesecond winding of the variable inductor with the electronic controlsystem in response to the calculating the desired resonant tankimpedance. In certain forms, the electronic control system is structuredto operate the first switching device by opening and closing theswitching device at resonant frequency so as to provide a resonantcurrent to the second input/output terminal. In certain forms, themethod comprises operating circuitry including a third winding woundaround the second core and coupled with the electronic control system;calculating a change in resonant tank impedance using the voltage andload requirements; and providing a current to the third winding of thevariable inductor in response to the calculating the desired resonanttank impedance. In certain forms, the electronic control systemcomprises a plurality of microprocessor based controllers. In certainforms, the resonant converter is structured and controlled to output ACpower.

It is contemplated that the various aspects, features, processes, andoperations from the various embodiments may be used in any of the otherembodiments unless expressly stated to the contrary. Certain operationsillustrated may be implemented by a computer executing a computerprogram product on a non-transient computer readable storage medium,where the computer program product includes instructions causing thecomputer to execute one or more of the operations, or to issue commandsto other devices to execute one or more operations.

While the present disclosure has been illustrated and described indetail in the drawings and foregoing description, the same is to beconsidered as illustrative and not restrictive in character, it beingunderstood that only certain exemplary embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the present disclosure are desired to be protected. It shouldbe understood that while the use of words such as preferable,preferably, preferred or more preferred utilized in the descriptionabove indicate that the feature so described may be more desirable, itnonetheless may not be necessary and embodiments lacking the same may becontemplated as within the scope of the present disclosure, the scopebeing defined by the claims that follow. In reading the claims, it isintended that when words such as “a,” “an,” “at least one,” or “at leastone portion” are used there is no intention to limit the claim to onlyone item unless specifically stated to the contrary in the claim. Theterms “coupled to,” “coupled with” and the like include indirectconnection and coupling and further include but do not require a directcoupling or connection unless expressly indicated to the contrary. Whenthe language “at least a portion” and/or “a portion” is used the itemcan include a portion and/or the entire item unless specifically statedto the contrary.

What is claimed is:
 1. A system comprising: a bidirectional resonantconverter comprising an input/output terminal, a switching devicecoupled with the input/output terminal, a resonant circuit coupled withthe switching device and including a variable inductor, an output/inputterminal coupled with the resonant circuit, and a DC biasing circuitoperatively coupled with the variable inductor; wherein the variableinductor comprises a toroidal core, a first winding wound around thetoroidal core and coupled with the switching device and the output/inputterminal, a second core structured to overlap a portion of the toroidalcore, and a second winding wound around the second core and coupled withthe DC biasing circuit; and wherein the DC biasing circuit iscontrollable to vary the inductance of the variable inductor bysaturating a portion of the toroidal core with magnetic flux.
 2. Thesystem of claim 1 wherein the second core comprises a cylindricalportion positioned within the center of the toroidal core and top andbottom plate portions extending radially outward to overlap a portion ofthe toroidal core.
 3. The system of claim 1 wherein the second corecomprises a C-shaped structure including a top portion overlapping thetoroidal core, a middle portion around which the second winding iswould, and a third portion overlapping the toroidal core.
 4. The systemof claim 1 wherein the variable inductor comprises a third corestructured to overlap a second portion of the toroidal core, and a thirdwinding wound around the third core and coupled with a second DC biasingcircuit, the second DC biasing circuit being controllable to vary theinductance of the variable inductor by saturating a second portion ofthe toroidal core with magnetic flux.
 5. The system of claim 1 wherein afirst flux path through the toroidal core for the first winding isgenerally toroidal and a second flux path through the toroidal core forthe second winding is perpendicular to the first flux path.
 6. Thesystem of claim 1 wherein the toroidal core comprises an air gappositioned such that the air gap and the first winding overlap along aportion of the toroidal core.
 7. A variable inductor comprising: atoroid comprising a first external surface; a first winding wound aroundat least a portion of the first external surface, the winding beingcoupled to an AC power source to generate AC flux in an AC flux path; acore comprising a second external surface; and a second winding woundaround the second external surface of the core, coupled to a controllerproviding DC power to the second winding, and structured to generate DCflux in a DC flux path, the DC flux path passing through a portion ofthe toroid, wherein the presence of DC flux generated by the secondwinding alters an inductive value of the inductor.
 8. The inductor ofclaim 7, wherein the core is structured to be selectively moveablerelative to the toroid effective to selectively vary the portion of thetoroid through which the DC flux passes.
 9. The inductor of claim 7,wherein the controller is structured to selectively saturate the AC fluxpath by providing DC power to the second winding.
 10. The inductor ofclaim 7, wherein the core comprises a cylinder with a first end and asecond end, a top plate coupled to the first end of the core and incontact with the first external surface of the toroid, and a bottomplate coupled to the second end of the core and in contact with thefirst external surface of the toroid.
 11. The variable inductor of claim10 comprising a second core having a cylinder with a third externalsurface, a first end, and a second end, a second DC winding wound aroundthe third external surface of the cylinder of the second core, a secondtop plate coupled to the first end of the cylinder of the second coreand in contact with the first external surface of the toroid, and asecond bottom plate electrically coupled to the second end of thecylinder of the second core and in contact with the first externalsurface of the toroid.
 12. The inductor of claim 7, wherein a portion ofthe core is encircled by the toroid.
 13. The inductor of claim 7,wherein the toroid is structured as one of a torus with a circular crosssection or a hollow square section ring with a rectangular crosssection.
 14. The inductor of claim 7, wherein the core comprises aC-core.
 15. A method comprising: operating resonant converter circuitryincluding a first switching device operatively coupled with a firstinput/output terminal and a resonant tank, the resonant tank including avariable inductor, the variable inductor including a first core, a firstwinding wound around the first core and operatively coupled with thefirst switching device, a second core, and a second winding wound aroundthe second core, a second switching device operatively coupled with thefirst winding of the resonant tank and a second input/output terminal,and an electronic control system operatively coupled with the firstswitching device and the second switching device and structure tocontrol current to the second winding; operating the electronic controlsystem to provide a current through the first winding of the variableinductor, provide a current through the second winding of the variableinductor so as to alter the inductance of the variable inductor, providepower to the second switching device from the resonant tank such thatthe voltage and current applied to second switching device by theresonant tank are with a current and voltage substantially equal to thecurrent and voltage provided to the second switching device by thesecond input/output terminal, and open the second switching deviceduring the zero-voltage, zero-current condition.
 16. The method of claim15 comprising: receiving voltage and load current requirements with theelectronic control system; calculating with the electronic controlsystem a desired resonant tank impedance using the voltage and loadcurrent requirements; and operating the electronic control system toprovide a current to the second winding of the variable inductor withthe electronic control system in response to the calculating the desiredresonant tank impedance.
 17. The method of claim 16 wherein theelectronic control system is structured to operate the first switchingdevice by opening and closing the switching device at resonant frequencyso as to provide a resonant current to the second input/output terminal.18. The method of claim 17 comprising: operating circuitry including athird winding wound around the second core and coupled with theelectronic control system; calculating a change in resonant tankimpedance using the voltage and load requirements; and providing acurrent to the third winding of the variable inductor in response to thecalculating the desired resonant tank impedance.
 19. The method of claim15 wherein the electronic control system comprises a plurality ofmicroprocessor based controllers.
 20. The method of claim 15 wherein theresonant converter is structured and controlled to output AC power.